Electronic, optoelectronic and micromechanical components based on group III element nitrides (referred to as “III-N materials”) on a silicon substrate represent considerable potential.
In particular, there is a large market for electronic power components based on semiconductors with a large band gap.
Indeed, the use of large band gap semiconductors may help reduce the size and complexity of the electronic circuits wherein they are substantially integrated.
A specific particularly promising application of these materials relates to Schottky type rectifier diodes, suitable for significantly reducing diode-related losses.
Indeed, it is estimated that a motor control unit using silicon-based rectifier diodes suffer from diode-related losses of the order of 2%, whereas the same unit using large band gap semiconductor-based diodes (such as SIC, GaN, etc.) only have diode-related losses of the order of 0.2%.
In the case of the substrate supporting the large band gap semiconductor, to optimize the cost of the component, it should be available in a large size (typically greater than or equal to 6 inches (150 mm)) and have a reasonable cost.
In this respect, silicon is one of the preferred materials due to the low cost thereof, the availability thereof, and the suitability thereof for semiconductor processing methods, which are standardized.
On the other hand, gallium nitride, which is, in theory, the ideal substrate for the epitaxial growth of III-N materials, is not currently available in bulk form under industrially viable conditions (the substrates being too small (i.e., not more than 2 inches (50 mm)) and having an excessively high cost).
Sapphire and silicon carbide are other potentially advantageous candidates, but are too expensive and subject to stock shortages, respectively.
Moreover, epitaxial GaN on sapphire display defects referred to as “micropipes.” These defects, caused by the formation of screw dislocation during material growth, typically have a diameter of the order of 250 to 500 nm. A density of these defects of the order of 3 to 6·105 cm−2 has thus been identified.
However, the silicon substrate, in spite of the advantages mentioned above, has two main drawbacks.
The first is a significant lattice mismatch with respect to III-N materials.
Indeed, for the Si(111) face, the lattice mismatch between GaN (wherein the lattice parameter is 3.189 Å) and Si (the lattice parameter is 3.840 Å) is 16.9%.
Secondly, there is a significant mismatch between the coefficients of thermal expansion of silicon and III-N materials.
In this way, the coefficient of thermal expansion of GaN is 5.59·10−6 K−1 whereas that of silicon is 2.59·10−6 K−1, representing a thermal mismatch of 53.7%.
Moreover, the divergent contraction of the silicon substrate (which is slow), and that of the III-N epitaxial layers (which is fast), on returning to ambient temperature after epitaxy, results in the layers being placed under tensile strain of +1.4 GPa at ambient temperature.
The lattice parameter mismatch is the source of crystalline defects in the III-N material, which are potentially harmful for component performances (leakage currents, ageing, etc.).
Thermal mismatch, for its part, is the source of cracking formed in the III-N material to relax the strain.
Cracks are macroscopic defects of the III-N material layer, which, due to the discontinuity caused by these cracks on the surface of the GaN layer, are unsuitable for component operation.
To try to remedy these mismatches between the silicon substrate and the epitaxial layer(s) of III-N material, it is known to form, as illustrated in FIG. 1, on a silicon substrate 1, a so-called buffer layer 2 of AlN and grow, by means of epitaxy, a layer 3 of GaN on the buffer layer 2.
Indeed, due to the lattice parameters of GaN (which, as mentioned above, is 3.189 Å) and AlN (which is of the order of 3.112 Å), the layer of GaN is subject to compressive strain when grown epitaxially on a layer of AlN.
In theory, i.e., if GaN grew pseudomorphically on AlN, the compression of GaN could be up to −10.9 GPa, and thus compensate for the tensile strain generated on returning to ambient temperature by the difference in coefficients of thermal expansion between GaN and the silicon substrate.
In fact, due to the high lattice mismatch between GaN and AlN (2.47%), GaN does not grow pseudomorphically but relaxes partially by forming dislocations and/or bending existing dislocations.
As a result, only a portion of the compressive strain generated at the GaN on the AlN interface can be maintained in the layer of GaN and is thus not sufficient to compensate for the tensile strain created during cooling.
In this way, the limit thickness of crack-free GaN in such a structure is around 1 μm, which is too low for most target applications.
To improve retention of the compressive strain in the layer of GaN, various teams have proposed forming, between the buffer layer and the layer of GaN, one or a plurality of “intermediate” layers.
A first process is that of the epitaxial growth of one or a plurality of layers of AlxGa1-xN (where 0<x<1) suitable for better retention of compression by graduating the aluminium content between the layer of AlN and the final layer of GaN.
One can refer to the works by H. Ishikawa, G. Y. Zhao, N. Nakada, T. Egawa, T. Soga, T. Jimbo, M. Umeno, High-quality GaN on Si substrate using AlGaN/AlN intermediate layer, Phys. Stat. Sol. A 176, 599 (1999), and U.S. Pat. No. 6,617,060 proposing the insertion, between the buffer layer of AlN and the useful layer of GaN, of a transition layer, wherein the Al composition decreases gradually on approaching the interface with the layer of GaN.
Alternatively, the transition layer may consist of a stack of layers wherein the Al content progressively decreases discretely.
The study by M. Hiberlen, D. Zhu, C. McAleese, M. J. Kappers, C. J. Humphreys, Dislocation reduction in MOCVD grown GaN layers on Si(111) using two different buffer layer approaches, 13th European Workshop on Metalorganic Vapor Phase Epitaxy (EWMOVPE-XIII), Ulm, Germany, Jun. 7-10, 2009 (B-11), shows that, compared to a continuous variation in the aluminium content, a discrete variation makes it possible to reduce the dislocation density within the final layer of GaN significantly (of the order of 108 cm−2 as opposed to 109 cm−2).
U.S. Pat. Nos. 6,649,287; 7,247,889; 7,339,205; 7,352,015; and 7,352,016 describe similar transition layers to that described above.
However, the literature relating to this type of structure, supported by experiments conducted by the inventors, demonstrates that it makes it possible to obtain a continuous crack-free layer of GaN wherein the thickness is not more than 2.5 μm.
This thickness remains too small for most of the target applications.
A second type of solution is the epitaxy of an alternation of layers of AlN and layers of GaN.
WO 01/95380 thus proposes, as illustrated in FIG. 2, a structure successively comprising a silicon substrate 1, a buffer layer 2 of AlN, a first layer 3a of GaN, an intermediate layer 4 of AlN and the useful layer 3 of GaN.
The intermediate layer 4 is monocrystalline and has a lattice parameter less than that of the layer 3 of GaN, enabling the compression of the overlying layer 3 of GaN during epitaxy with a view to compensating at least partially for the tensile strain generated during cooling.
Such a structure makes it possible to obtain a crack-free layer 3 of approximately 3 to 4 μm, which is significant but too small for some target applications.
FIG. 3 shows an alternative embodiment of this structure, which successively comprises a silicon substrate 1, a buffer layer 2 of AlN, a first layer 3a of GaN, a first intermediate layer 4a of AlN, a second layer 3b of GaN, a second intermediate layer 4b of AlN and a useful layer 3 of GaN.
Inserting the second intermediate layer 4b of AlN in the structure makes it possible to increase the thickness of the useful layer 3 of GaN, but to an extent that remains insufficient for some of the target applications.
The aim of the invention is thus that of defining a method for manufacturing by means of epitaxy, a crack-free layer of GaN that is thicker than the layers obtained to date.
More specifically, the aim of the invention is that of producing a continuous useful layer of GaN (i.e., in one piece, not containing any layer of a material other than GaN) having a thickness greater than 5 μm and having a dislocation density less than or equal to 5·108 cm−2 on a substrate optionally greater than or equal to 6 inches (150 mm) in diameter.